Recently, in a wireless communications field, studies have been made concerning multiple-input multiple-output (MIMO) that allows throughput to be improved by providing a plurality of antennas on both a transmitter and a receiver. A transmitter for MIMO transmits different data from a plurality of transmitting antennas at the same time. This makes it theoretically possible to transmit data in multiples of the number of transmitting antennas comparing with a transmitter of a single transmitting antenna. A receiver for MIMO receives data by a plurality of receiving antennas and separates from one another the data transmitted from the respective transmitting antennas by carrying out, for example, a matrix operation.
In other words, in MIMO, the data transmitted from the transmitting antennas are propagated through channels of different characteristics, and the data propagated through each of the channels are combined and are received by each of the receiving antennas of the receiver. Therefore, the receiver uses the combined data received by each of the receiving antennas to separate from one another the data for each of the transmitting antennas. In this case, for the receiver to correctly separate the data transmitted from each of the transmitting antennas, it is desirable that the deterioration in reception quality by channel characteristics be compensated.
Accordingly, it has been studied for the transmitter to perform precoding that applies in advance inverse characteristics of the channel characteristics to the data to be transmitted from each of the transmitting antennas. Specifically, when channel state information (CSI) indicative of channel characteristics is fed back from the receiver to the transmitter, the transmitter performs the precoding using a codebook so as to compensate the deterioration in reception quality attributable to the channel characteristics estimated from the CSI.
However, an error rate in the receiver may not be sufficiently reduced by the compensation for deterioration in reception quality using the precoding alone, and thus an improvement of throughput by MIMO is difficult. In other words, even when the transmitter transmits data different from one another from the multiple transmitting antennas at the same time, unless the error rate in the receiver is sufficiently reduced, retransmission of data occurs frequently. As a result, the transmitter ends up transmitting the same data a number of times and, not only is throughput not improved, but it may adversely lower the throughput by adopting MIMO.